Carbon nanotubes (CNTs, aka fullerene pipes) are nanoscale carbon structures comprising graphene sheets conceptually rolled up on themselves and closed at their ends by fullerene caps. Single-walled carbon nanotubes (SWNTs) comprise but a single such graphene cylinder, while multi-walled nanotubes are made of two or more concentric graphene layers nested one within another in a manner analogous to that of a Russian nesting doll. Since their initial preparation in 1993 (Iijima et al., Nature, 1993, 363, 603; Bethune et al., Nature, 1993, 363, 605; Endo et al., Phys. Chem. Solids, 1993, 54, 1841), SWNTs have been studied extensively due to their unique mechanical, optical, electronic, and other properties. For example, the remarkable tensile strength of SWNTs has resulted in their use in reinforced fibers and polymer nanocomposites (Zhu et al., Nano Lett. 2003, 3, 1107 and references therein). For other existing and potential applications of CNTs, see Baughman et al., Science, 2002, 297, 787-792.
SWNTs normally self-assemble into aggregates or bundles in which up to several hundred tubes are held together by van der Waals forces. For many applications, including biomedical ones, the separation of individual nanotubes from these bundles is essential. Such separation improves the dispersion and solubilization of the nanotubes in the common organic solvents and/or water needed for their processing and manipulation. Covalent modifications of the SWNT surface generally help to solve this problem by improving the solubility/suspendability and processability of the nanotubes. While chemical functionalizations of the nanotube ends generally do not change the electronic and bulk properties of these materials, sidewall functionalizations do significantly alter the intrinsic properties of the nanotubes (Chen et al., Science, 1998, 282, 95-98; Mickelson et al., Chem. Phys. Lett., 1998, 296, 188-194) and typically have a more profound impact on their solubility/suspendability (Boul et al., Chem. Phys. Lett., 1999, 310, 367-372). However, the extent of documented results in this new field of chemistry is limited, primarily due to the current high cost of the nanotubes.
Additional challenges faced in the modifications of SWNT sidewalls are related to their relatively poor reactivity—largely due to a much lower curvature of the nanotube walls relative to the more reactive fullerenes (M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996, Vol. 1), and to the growing strain within the tubular structure with increasing number and size of functional groups attached to graphene walls. The sp2-bonding states of all the carbon atoms comprising the nanotube framework facilitate the predominant occurrence of addition-type reactions. The best characterized examples of these reactions include additions to the SWNTs of nitrenes, azomethine ylides and aryl radicals generated from diazonium salts (V. N. Khabashesku, J. L. Margrave, Chemistry of Carbon Nanotubes in Encyclopedia of Nanoscience and Nanotechnology, Ed. H. S. Nalwa, American Scientific Publishers, 2004; Bahr et al., J. Mater. Chem., 2002, 12, 1952; Holzinger et al., Angew. Chem. Int. Ed., 2001, 40, 4002). Other reported sidewall functionalizations of SWNTs involve organic radicals (Peng et al., Chem. Commun., 2003, 362; Ying et al., Org. Lett., 2003, 5, 1471; Peng et al., J. Am, Chem. Soc., 2003, 125, 15174) and the Bingel reaction (Coleman et al., J. Am. Chem. Soc., 2003, 125, 8722). Additionally, the addition of hydrogen to the sidewalls of SWNTs has been reported to occur under conditions of the Birch reduction (Pekker et al., J. Phys. Chem. B, 2001, 105, 7938).
The diameter and chirality of individual CNTs are described by integers “n” and “m,” where (n,m) is a vector along a graphene sheet that is conceptually rolled up to form a tube. When |n−m|=3q, where q is a non-zero integer, the CNT is a semi-metal (bandgaps on the order of milli eV). When n−m=0, the CNT is a true metal with a bandgap of 0 eV, and referred to as an “armchair” nanotube. All other combinations of n−m are semiconducting CNTs with bandgaps typically in the range of 0.3 to 1.0 eV. See O'Connell et al., Science, 2002, 297, 593. CNT “type,” as used herein, refers to such electronic types described by the (n,m) vector (i.e., metallic, semi-metallic, and semiconducting). CNT “species,” as used herein, refers to CNTs with discrete (n,m) values. CNT “composition,” as used herein, refers to make up of a CNT population in terms of nanotube type and species.
All known CNT preparative methods lead to polydisperse CNT populations of semiconducting, semimetallic, and metallic electronic types. See M. S. Dresselhaus, G. Dresselhaus, P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, Academic Press, San Diego, 1996; Bronikowski et al., Journal of Vacuum Science & Technology A 2001, 19, 1800-1805; R. Saito, G. Dresselhaus, M. S. Dresselhaus, Physical Properties of Carbon Nanotubes, Imperial College Press, London, 1998. As such, a primary hurdle to the widespread application of CNTs, and SWNTs in particular, is their manipulation according to electronic structure (Avouris, Acc. Chem. Res. 2002, 35, 1026-1034). Recently, however, methods to selectively functionalize CNTs based on their electronic structure (i.e., electronic type) have been reported (Strano et al., Science, 2003, 301, 1519-1522; commonly assigned co-pending International Patent Application PCT/US04/24507, filed Jul. 29, 2004). In such reports, metallic CNTs are seen to react preferentially with diazonium species, permitting a separation or fractionation of metallic (including semi-metallic) and semiconducting CNTs via partial functionalization of a mixture of metallic and semiconducting CNTs. For a detailed discussion of CNT types and species, and their optical identification, see Bachilo et al., Science, 2002, 298, 2361-2366; and Weisman et al., Nano. Lett., 2003, 3, 1235-1238.
Despite such above-described advances in the chemically derivatizing the sidewalls of carbon nanotubes, most such processes require ultrasonication of the carbon nanotubes during the derivatization process. This sonication can potentially damage many of the nanotubes in the sample. Thus, a method of derivatizing carbon nanotubes under gentler conditions would be very beneficial.